The present invention relates to a mass spectrometer and a method of mass spectrometry. The preferred embodiment relates to a method of enhancing the duty cycle of an orthogonal acceleration Time of Flight mass analyser.
In a conventional orthogonal acceleration Time of Flight mass analyser ions having approximately the same energy are arranged to be passed through an orthogonal acceleration region. An orthogonal acceleration electric field is periodically applied across the orthogonal acceleration region in order to orthogonally accelerate ions into the drift region of the Time of Flight mass analyser. The length of the region over which the orthogonal acceleration electric field is applied, the energy of the ions and the frequency of the application of the orthogonal acceleration electric field determine the sampling duty cycle of the Time of Flight mass analyser. Ions which have approximately the same energy but different mass to charge ratios will have different velocities and hence will have different sampling duty cycles.
The maximum ion sampling duty cycle for a conventional orthogonal acceleration Time of Flight mass analyser when used with a continuous ion beam is typically approximately 20-25%. The maximum duty cycle is achieved for those ions which have the maximum mass to charge ratio which are mass analysed by the mass analyser. The ion sampling duty cycle is lower for ions having relatively low mass to charge ratios.
If ions having the maximum mass to charge ratio which can be mass analysed by the mass analyser have a mass to charge ratio mo and the sampling duty cycle for these ions is DCo then more generally the sampling duty cycle DC for ions having a mass to charge ratio m is given by:
                              D          ⁢                                          ⁢          C                =                  D          ⁢                                          ⁢                      C            0                    ⁢                                    m                              m                0                                                                        (        1        )            
It can be shown that the average sampling duty cycle DCav is equal to two thirds of the maximum sampling duty cycle DC0. Accordingly, if the maximum sampling duty cycle is 22.5% then the average sampling duty cycle is 15%.
It is known to attempt to improve the duty cycle just for ions having a relatively narrow range of mass to charge ratios by trapping and releasing ions from an ion storage device which is arranged upstream of the Time of Flight mass analyser. An orthogonal acceleration pulse is timed to coincide with the arrival of ions of interest at an orthogonal acceleration region adjacent the orthogonal acceleration electrode. If ions are stored in an ion trap upstream of the orthogonal acceleration Time of Flight mass analyser and are released in a series of packets rather than allowed to flow continuously, then the application of a pusher voltage to the orthogonal acceleration electrode can be synchronised with respect to the release of each packet of ions from the ion trap. According to this arrangement ions are arranged to be released from the ion trap with substantially constant energy. Ions having different mass to charge ratios will therefore travel towards the orthogonal acceleration region with different velocities. As a result, ions having different mass to charge ratios will arrive at the orthogonal acceleration region at different times. The time delay between the release of a packet of ions from the ion trap to the application of the pusher voltage to the orthogonal acceleration electrode determines the mass to charge ratio of the ions that are transmitted into the drift region of the orthogonal acceleration Time of Flight mass analyser. For those ions having a narrow range of mass to charge ratios which are transmitted into the draft region of the orthogonal acceleration Time of Flight mass analyser, the duty cycle can be increased to substantially 100%. However, the majority of other ions having other mass to charge ratios will not lie fully in the orthogonal acceleration region at the time when the pusher voltage is applied to the pusher electrode. Accordingly, all other ions will have substantially lower sampling efficiencies and ions having mass to charge ratios which are removed from those ions which are orthogonally accelerated will have a sampling efficiency of zero.
It is also known to attempt to increase the duty cycle of a Time of Flight mass analyser for ions having a limited range of mass to charge ratios by providing a travelling wave ion guide upstream of a mass analyser. The orthogonal acceleration voltage is sychronised with packets of ions released from the travelling wave ion guide. The ion guide is arranged to partition a continuous stream of ions into a series of packets of ions. The time delay between the release of a packet of ions from the exit region of the travelling wave ion guide to the application of a pusher voltage determines the mass to charge ratio range of ions which are transmitted into the drift region of the orthogonal acceleration Time of Flight mass analyser. For those ions that are transmitted the duty cycle can be increased to substantially 100%. However, ions having other mass to charge ratios will not all be present in the orthogonal acceleration region at the time when the pusher voltage is applied to the orthogonal acceleration electrode. Accordingly, the sampling efficiency for these ions will be lower and may be zero.
It is desired to provide an improved mass spectrometer and method of mass spectrometry.